This section is intended to introduce the reader to various aspects of art, which may be associated with embodiments of the present invention. This discussion is believed to be helpful in providing the reader with information to facilitate a better understanding of particular techniques of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not necessarily as admissions of prior art.
Hydrocarbons have been important to civilization for centuries. In the relatively recent past, the efforts to find, access, extract, and utilize hydrocarbons have become increasingly complicated. Present hydrocarbon recovery operations require drilling and completions equipment to reach far beneath the earth's surface and/or to reach across long horizontal distances. Those in the industry are ever more aware of the need for deepwater wells, extended reach wells, ultra-extended reach wells, and other drilling scenarios that push drilling, completions, and subsurface technologies to the extremes. For example, some reservoirs may not be deep or far from the rig, but nevertheless may be difficult to reach due to the types of formations through which the well must extend. The equipment costs, time, and labor costs associated with drilling a well for use in hydrocarbon recovery operations is extremely high in conventional wells. As the need for longer, deeper, and/or more complicated wells becomes more common, the increased costs are making many projects economically less advantageous. In some scenarios, the economics of accessing the reserves make the reserves inaccessible. Accordingly, while the hydrocarbon recovery industry has developed great technology in the past, there is a continuing need to improve upon the past technologies and to make step changes from conventional practices.
One challenge in drilling any well is maintaining the pressure in the annulus between the pore pressure gradient and the fracture gradient. As is well understood, the pore pressure gradient relates to the pressure of the formation fluids and is the pressure above which the annulus must be maintained to prevent formation fluids from entering the annulus. The fracture gradient relates to the strength or integrity of the formation and is the pressure below which the annulus must be maintained to prevent fracturing the formation. Maintaining the annulus pressure at or above the pore pressure gradient can be effective in minimizing the risk of blow-outs, kicks, and other problems associated with the unexpected inflow of formation fluids. Maintaining the pressure at or below the fracture gradient can be effective in minimizing the risk of fractures, lost returns, and other problems and complications associated with fracturing the formation. There are various reasons why the annulus pressure should be maintained between the pore pressure gradient and the fracture gradient; these are provided as merely illustrative examples. FIG. 1 presents one schematic representation of the pore pressure gradient (PPG) 102 and the fracture gradient (FG) 104 in a plot 100 of depth 106 against gradient 108. As can be seen in the plot 100, both the pore pressure gradient and the fracture gradient increase with the depth of the well.
The need to maintain the annulus pressure within these bounds has been understood for many years and several methods and tools have been developed to facilitate efforts to meet that need. One example of such methods and tools is the use of drilling muds customized for a particular well and/or for a particular interval of a well. Drilling muds of a variety of constitutions have been developed and utilized over the years, with the muds including a variety of additives. The variety of drilling muds and additives have been developed to provide muds of different densities, which is often referred to as the effective mud weight or density equivalent when taking into account the effects of the hydrostatic head. The selection of a mud having a particular mud weight allows the operator to customize the pressure of the annulus to correspond with the conditions in the open hole section of the wellbore. While this principle is quite conventional, FIGS. 1 and 2 help to illustrate at least one limitation of this conventional approach. FIG. 2 illustrates in plot 200 the density equivalent 208 (in pounds per gallon) of a modeled conventional mud 210 as a function of depth 206 (in 1000 meters). In FIG. 2, the conventional mud 210 is modeled in two different states, while circulating in the well 210′ and while static in the well 210″. As can be seen in FIG. 2, the equivalent density of the conventional mud does not change at a pace sufficient to follow the increase in the pore pressure gradient 202 or the fracture gradient 204. If the conventional mud 210 were pumped to drill in the exemplary well modeled in FIG. 2, it would have a suitable mud weight below 3500 meters (to some point not shown), but would fracture the well at depths shallower than 3500 meters.
The risk of fracturing the well at those more shallow depths, also referred to as upper intervals, is commonly addressed through the installation of casing in the well. As is well known, casing consists essentially of tubulars disposed in the well and cemented into place to provide a barrier between the well and the formation. The casing may include a variety of materials and may be installed in a multitude of manners depending on the condition of the well, the formation, etc. Once the casing is in place, the casing itself increases the structural integrity of the wellbore wall and retains the formation fluids behind the casing preventing kicks, blowouts, etc. FIG. 1 illustrates schematically the impact of casing in the well. Casing 112 is installed at various locations in the well effectively defining drilling intervals. Different conventional drilling muds 110 are used in each drilling interval, with each mud 110 having a different effective mud weight adapted for the particular interval. As can be seen in FIG. 1, the effective mud weight of the drilling mud 110 for a particular interval is conventionally adapted to correspond with the fracture gradient at the top of the interval and to more closely correspond with the pore pressure gradient at the bottom of the interval. As the density equivalent of the mud approaches the pore pressure gradient, the drilling is conventionally stopped and the recently drilled interval is cased before progressing to the next interval with a different drilling mud.
While casing can be used to overcome the pore-pressure-gradient/fracture-gradient (PPG/FG) problem once the casing is installed, it does not solve all of the problems. For example, casing is of little value when actively drilling. This is illustrated in FIG. 1 by the numerous casing strings that must be installed and the different types of muds required to drill to the desired depth. It can be readily appreciated that the drilling operations are stopped while casing is run into the well and installed in the desired position. The time lost in waiting for the drill tool to be tripped out of the well, the casing to be tripped into the well and cemented in place, and then to re-trip the drilling equipment into the well can significantly increase the time requirements for a drilling operation. As the manpower and equipment costs are generally time-based and expensive, the delays required to introduce casing into the well significantly increase the cost of drilling operations. While the additional time requirements of installing the casing are significant disadvantages, the casing presents still further issues that render them a less than desirable solution. As a simple example, the casing itself adds costs and its installation adds complexity to the well construction and to the completion and production of the well (for example, the inclusion of casing may require perforations to re-open the formation to the well annulus).
As a more involved, but equally readily appreciated example, the use of multiple casing strings has the impact of reducing the diameter of the well. Absent relatively complex and/or expensive under-reaming operations and expandable casing, the casing strings installed in a well are typically in a nested arrangement. In such a nested arrangement, the casing for a lower interval of the wellbore is nested inside the casing for an upper interval. The nesting arrangement may require the well to be excessively large at the surface and/or may limit the depth of the well by decreasing the diameter of the well before the target depth is reached. This is particularly disadvantageous when the actual PPG/FG operating window is more narrow than expected resulting in a larger than expected number of distinct drilling intervals and casing intervals. With the telescoping nature of the casing intervals, the additional casing intervals may limit the total depth attainable and may prevent the drilling operation from reaching a target depth.
While operators in the hydrocarbon recovery industry, such as drilling companies, service companies, etc., have developed a number of drilling muds and mud additives, the conventional drilling muds are effectively static throughout their journey through the well (as illustrated in FIG. 2). While this is acceptable within the paradigm of conventional drilling operations, it can be associated with the need for shorter casing intervals, rapidly narrowing wellbore diameters, etc. In the recent past, several publications have suggested an alternative paradigm for drilling muds: drilling muds having a dynamic mud weight or density during the drilling operation. For example, and as is well explained in several earlier applications, drilling muds comprising compressible objects result in a drilling mud having a variable density or variable mud weight. It can be readily understood that if a plurality of compressible objects are reduced in size in response to increasing hydrostatic pressure, the mud weight would increase in correspondence with the number of objects undergoing compression and the relative degree of compression. More thorough descriptions of variable density drilling muds can be found in various prior publications, including: US Patent Publication Nos. 20070027036; 20090084604; 20090090558; 20090090559; and 20090091053, each of which is incorporated herein by reference in their entirety for all purposes. Additional discussion of drilling muds having a potentially variable density can be found in U.S. Pat. Nos. 7,108,066 and 7,482,309 and US Patent Publication Nos. 20050284641; 20050161262; 20050113262; 20060254775; 20090114450; and 20090163388.
FIG. 2 provides a schematic, representative illustration of this effect by illustrating the effective density 208 of a variable density mud 220, in both the circulation mode 220′ and the static mode 220″. As can be seen, the compressible objects can be adapted to provide a drilling mud having a variable density adapted to follow the pore pressure gradient and the fracture gradient. In some implementations, the variable density mud can be used to extend the length of a drilling interval sufficient to reduce the total number of casing strings (or the number of drilling intervals) resulting in cost and time savings and potentially deeper or longer wells. FIG. 3 illustrates the exemplary variable density drilling mud 320 in comparison to the convention drilling mud 310 and casing 312 combination. The remaining features in FIG. 3 are referenced by numbers corresponding to those of FIGS. 1 and 2. As can be seen in FIG. 3, for the segment illustrated, the variable density drilling mud 320, representatively, eliminates the need for as many as three casing strings.
As illustrated in FIG. 3 and the several prior patent applications referenced above, the potential benefits of using a variable density mud in hydrocarbon recovery operations are great in the context of drilling operations. Additionally, there may be various other beneficial uses of such a fluid or of fluids comprising compressible objects. Regardless of the end use of the compressible objects and/or muds comprising the same, it can be appreciated that the conditions under which these compressible objects must operate can be severe and can vary widely in the various environments in which hydrocarbon recovery operations occur. While the above-referenced publications disclose a variety of possible constructions and configurations for the compressible objects, it is presently believed that some of those proposed are better than others. Moreover, while various methods of making such compressible objects were disclosed in several of the above-referenced publications, suitable and/or optimal methods of manufacturing such compressible objects continue to evolve. For example, as the operating conditions under which the compressible objects will be used are better understood, the methods of making such compressible objects need to evolve to meet those conditions. Accordingly, the need continues to exist for improved methods of fabricating compressible objects suitable for use in hydrocarbon recovery operations, including drilling operations.
In at least some of the above-referenced publications, the present inventors disclose various properties of compressible objects suitable for such applications. For example, it was disclosed that the compressible objects could be provided in a variety of shapes but that spherical or spheroidal shapes were preferred over shapes having sharp angles. Similarly, it was disclosed that an oblate disk having a range of aspect ratios (major axis; minor axis) may be preferred to obtain certain desired properties, such as tensile strength, elasticity, etc. A variety of materials were disclosed for use in making such compressible objects and a variety of methods were similarly disclosed. For example, polymers and other materials were disclosed as possible wall materials, particularly polymers known for having high strength and low permeability properties, such as polyimides. Similarly, the present inventors previously disclosed methods of making such compressible objects from polymers and other materials including molding the objects, forming the objects from a plurality of component parts, and forming bubbles or droplets of the polymer.
Among the variety of polymers that may be used to provide a compressible objects, polyimides have been described as viable candidates. Polyimides are a class of polymers that have been known for decades. Polyimides are conventionally used in a limited sub-set of forms due to the complex chemistry and condition requirements involved in converting the polyamic acid to a polyimide, which may be referred to as the imidization reaction. It is generally recognized that the polyamic acid to polyimide reaction was discovered many years before the first workable or usable form of polyimide was developed due to the complexity of controlling the imidization reaction, of providing the proper reaction conditions while forming the polyimide product, etc. At the time the present technologies were developed, polyimides were available as films, membranes, foams, fibers, and coatings. Most available polyimides were generally thin, substantially two-dimensional products. Hollow polyimide fibers have been developed in the more recent past using certain polyamic acids and under conditions conducive to the formation of membranes, which is the primary use of such fibers. As is generally understood, the final polyimide product may have vastly different properties depending on the conditions of the reaction and depending on the polyamic acid selected. Accordingly, while a particular fabrication technique may be suitable for a given polyamic acid being imidized for forming a membrane, the same fabrication technique may be inapplicable if attempting to imidize the same polyamic acid into an impermeable barrier film.
From the above discussion, it can be understood that compressible objects have applicability in hydrocarbon recovery operations at least as a component in drilling muds. Moreover, it can be understood that while any compressible object may be used to provide some value, optimized compressible objects will be suited for the conditions in which they will be used, such as to enable recycle and reuse of the compressible objects during one or more drilling operations. In addition, high mechanical integrity is required to predictability compress, deform and subsequently decompress and re-establish the bead configuration. Still further, while several variations of compressible objects have been proposed in the above-referenced publications, methods of forming the compressible objects to have desired configurations and properties are still needed. The present disclosure provides methods of fabricating compressible polyimide objects and discloses compressible polyimide beads made by such methods. More particularly, the present disclosure provides methods of fabricating compressible polyimide objects suitable for use in hydrocarbon recovery operations.